Retroviral vectors derived from C-type murine leukaemia viruses (MLVs) have emerged as highly versatile gene delivery vehicles and have been selected for use in many human gene therapy protocols, especially those requiring transduction of normal or neoplastic haemopoietic cells. In the interests of safety and efficacy, particularly for direct genetic modification of target cells in vivo, it is desirable that retroviral gene delivery should be accurate but the clinically approved (amphotropic) retroviral vectors in current use attach to an ubiquitously expressed cellular receptor and therefore infect human cells promiscuously. Cell-selective retroviral gene delivery might be achieved by modification of the viral membrane spike glycoproteins responsible for receptor-mediated virus entry.